Haematobia irritans (horn fly) is a bloodsucking insect that affects grazing cattle. Since this fly's introduction into Uruguay in 1992, pest management practices used to control the insect have been exclusively based on the use of insecticides, which has caused synthetic pyrethroid resistance. The use of insecticides is a major constraint to livestock production due to food safety and environmental concerns. This study evaluated the effectiveness of a walk-through fly trap for horn fly control. The field trial was conducted in 18 trail evaluations dates from 2015 to 2016 in Holstein-Frisian lactating cows on two dairy farms in southern Uruguay. The traps were placed at the exit of the milking parlor. Two digital cameras were used to record video at the entrances and exits of the traps. On each of the 18 trail evaluation, between 30 and 158 cows were randomly selected for fly counting according to video records. On Farm 1, a total of 718 cows were assessed. The median number of flies per cow at the entrance of the traps was 22 (ranging from 1 to 199), while the median number at the exit was three flies per cow (ranging from 0 to 22). The median efficiency of the trap was 88%. Farm 2 had 345 observations, and the median fly count at the entrance of the traps was of 22 flies per cow (ranging from 1 to 129) and four flies at the exit (ranging from 0 to 35) with a median efficiency of 82%. It was observed that the effectiveness of the fly traps varied depending on the number of flies at entry, the season of the year and the farm site. In conclusion, fly traps could be used for the control of H. irritans in milking cows without the use of insecticides.